The present invention relates to injection blow molding apparatus and techniques, and more particularly, is concerned with method and apparatus which cause a parison to be stretched or rotated as it is moved between the parison and blow molds.
In the blow molding of articles from a plasticized material, it is difficult to produce an article such as a bottle having a relatively large height compared to its diameter or finish unless the parison mold has a similar dimensional ratio. Also, forming a parison on a pin in the parison mold with a relatively high length to diameter ratio requires accurate positioning of the pin within the cavity of the parison mold, otherwise, nonuniform wall thicknesses may be produced and such nonuniformity may be intolerable in thin walled bottles or tubes.
It is also desirable to control the molecular arrangement or orientation in the finished article in order to develop maximum strength. Such control can be exercised by appropriate stretching and twisting of the parison while the settable material forming the parison is still in a semi-fluid state at specific temperatures depending upon the material being used.
Both of the above problems may be solved in accordance with the present invention by stretching a relatively short parison axially of itself prior to the blowing operation so that an elongated parison is placed in the blow mold with the molecular chains generally aligned with the direction in which the parison is being stretched. Simple stretching of the parison prior to or during a blowing operation is taught in U.S. Pat. Nos. 2,834,154 and 3,244,778. In the prior art patents, however, the specific apparatus employed during the formation of the parison is relatively unique and hence, not adaptable to the more conventional injection blow molding machines in which a parison is completely formed in a parison mold and then is transferred to the blow mold for expansion to its final form.
Additionally, it is desirable to control the orientation of the molecular chains in the plastic material more precisely than that which occurs when the parison is merely stretched axially of itself. Control of the temperature of the parison and the twisting of the parison enable the molecular arrangement in the finished article to be further regulated for a broader choice of results.
Accordingly, it is an object of the present invention to provide a method and apparatus by which the molecular arrangement of a plastic article may be controlled in an injection blow molding machine. It is a further object of the invention to provide a method and apparatus for forming bottles and similar articles which have relatively large heights compared to their diameters or finishes.